1. Field of the Invention
The present invention relates to a rotary cooling roller used for e.g. sheet producing apparatus, sheet processing apparatus or sheet laminating apparatus.
2. Description of the Related Art
A conventional rotary cooling roller is disclosed in JP-A-63 (1988)-282393 for example. The conventional roller is incorporated in a film-laminating apparatus in which a thin film of synthetic resin is laminated on elongated, continuous paper (base paper). The laminated resin film and the base paper are squeezed between the cooling roller and a presser roller facing the cooling roller. Thus, the resin film, cooled by the cooling roller, is fixed to the base paper, thereby providing product laminate paper coated with the resin film.
The conventional cooling roller includes a cylindrical body, two circular end plates fixed to both ends of the cylindrical body, and a plurality of cooling tubes arranged inside of the cylindrical body. Each of the end plates is provided with a supporting shaft protruding outward from the center of the end plate. The cooling roller is rotatably supported, via these supporting shafts, by a fixing member. Inside of the cylindrical body is contained a volatile working fluid for cooling the surface of the cylindrical body. Each of the cooling tubes is designed to pass cooling water for cooling the evaporated working fluid.
In operation, the facing presser roller is pressed against the cooling roller, thereby exerting a rather great pressing force (about 2-10 kg/mm) on the cooling roller. The pressing force necessary to laminate the resin film on the base paper depends upon e.g. the kind of laminate paper to be produced and/or production speed of the laminate paper. Specifically, a greater pressing force is needed, for example when a flatter surface is desired in the resulting laminate paper, or when a faster production speed is desired. In the latter case, the pressing force needs to be increased because it is necessary to prevent air from being trapped between the laminated resin film and the base paper.
The conventional cooling roller has been found disadvantageous in the following points.
As stated above, the cooling roller is supported via the supporting shafts protruding from the ends plates of the roller. Since these two supporting shafts are widely spaced in the longitudinal direction of the roller, the cooling roller as a whole tends to be arched upon receiving a pressing force exerted by the presser roller. In addition, since the conventional cooling roller is not provided with any reinforcing member, the surface of the cooling roller may be partially dented when the presser roller is forcibly pressed against the cooling roller. Under these adverse circumstances, the resulting laminate paper may often suffer a defect such as incomplete adhesion of the resin film to the base paper. This situation will be aggravated when the length and/or diameter of the cooling roller is rendered greater.
The above problem may be addressed by increasing the thickness of the cylindrical body. Specifically, supposing that the cylindrical body is made of steel and has an outer diameter of 900 mm and a length of 3000 mm, the wall thickness of the cylindrical body may need to be about 18 mm for a pressing force of 2 kg/mm or 32 mm for a pressing force of 7 kg/mm.
However, a greater thickness of the cylindrical body is disadvantageous in facilitating the heat conduction from the outer surface to the inner surface of the cylindrical body. For instance, in order to lower the temperature of the cylindrical body by 1xc2x0 C. per unit wall thickness (1 mm), the temperature difference between the outer and the inner surfaces of the cylindrical body may need to be 18xc2x0 C. for the wall thickness of 18 mm and 32xc2x0 C. for the wall thickness of 32 mm. This means that a thicker cylindrical body needs a colder coolant to be held in contact with the inner surface so that the temperature of the resin film is lowered by a given amount. Disadvantageously, the preparation of such a colder coolant may lead to an increase in production cost.
To avoid such a cost increase, the operating speed of the sheet laminating apparatus may be slowed down, so that the heat conduction from the resin film to the cylindrical body can be performed over a sufficiently long period of time. Disadvantageously, however, this alternative compromises the production efficiency.
The present invention has been proposed under the circumstances described above, and its object is to provide a cooling roller boasting a sufficient mechanical strength as well as good thermal conductivity.
According to the present invention, there is provided a rotary cooling roller which includes an outer cylinder having first and second ends, first and second end plates for respectively closing the first and the second ends of the outer cylinder, a supporting shaft protruding outward from a center of each end plate, volatile working fluid contained in the outer cylinder, cooling tubes for passing a coolant, an inner cylinder arranged in the outer cylinder to surround the cooling tubes and having first and second ends connected to either of the end plates and the supporting shaft, a cylindrical space defined between an outer surface of the inner cylinder and an inner surface of the outer cylinder, and a spacer arranged in the cylindrical space so that a pressing force exerted on the outer cylinder and directed toward the supporting shaft is passed to the inner cylinder. The inner cylinder is formed with a plurality of through-holes for causing the cylindrical space to communicate with an inner space of the inner cylinder.
With such an arrangement, the inner cylinder can mechanically support the outer cylinder upon which an external force is exerted. Thus, the thickness of the outer cylinder does not need to be unduly great, which is advantageous to facilitating the heat conduction from a warmer sheet object (such as extruded resin film) to the outer cylinder.
Preferably, the cooling roller may further include a circular reinforcing plate arranged at a predetermined location in an axial direction of the inner cylinder. The reinforcing plate may have a circumferential surface held in contact with an inner surface of the inner cylinder. The reinforcing plate may be fixed to the supporting shaft. Alternatively, the reinforcing plate may be spaced from the supporting shaft.
In a preferred embodiment of the present invention, the spacer may extend circumferentially of the inner cylinder.
Preferably, the spacer may be formed with a cutout to allow the working fluid to flow back and forth between two opposite sides of the spacer. The cutout may be a through-hole extending through the spacer.
Preferably, the cooling roller may further include a first coolant chamber adjacent to the first end plate and a second coolant chamber adjacent to the second end plate. In this case, the supporting shaft extends through the first and the second coolant chambers.
Preferably, the supporting shaft may be provided with a coolant inlet passage and a coolant outlet passage which communicate with the first and the second coolant chambers, respectively, via through-holes formed in the supporting shaft.
Preferably, the outer cylinder may be smaller in thickness but greater in length than the inner cylinder.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.